Coaxial line termination



April 7, 1959 E. M. LEYhroN 2,881,399:

CQAXIALl LINE TERMINATION Filed Dec. 1, 195s Fgj Mm? am' INVENTOR. ERICM. LEYTDN FITTo A/Ey Uairdswrshf 2,881,399 y COAXIAL LINE TERMINATION Eric M. Leyton, Princeton, NJ., assigner to Radio Corporation of America, a corporation of Delaware Application December 1, 1953, serial No. 395,477

s claims. (cl. ssa-zz) This invention relates to a coaxial line termination or load, and more particularly to a termination wherein large amounts of radio frequency power may be dissipated in a circulating liquid such as ordinary tap water.

One application for the coaxial line termination or load lof the present invention is for usel as a dummy load connected to the output of a radio transmitter while testing and tuning the radio transmitter. The output of the radio transmitter is normally connected to a radiating antenna, but it is often desirable to substitute a dummy load for the antenna when adjustments are made to the transmitter. The coaxial line termination of this invention is generally useful whenever it is desired to dissipate radio frequency energy. The termination is also useful in conjunction with measuring apparatus in determining the amount of radio frequency energy supplied by a source to the termination.

The dissipation of large amounts of radio frequency energy results in the generation of a large amount of heat which must be carried away. VMany of the prior art terminations have necessarily included large capacity heat exchanger equipments by which the heat generated in the load or termination is carried away. In such devices, the inner or the outer conductor of the coaxial line termination includes resistive material and the heat generated therein is removed by the circulatinglliquid of aheat exchanger equipment. It 'is an .object"`ofy"this invention to provide an improved termination which is simpler and more effective than the prior art devices. 1

It is another object to provide a termination wherein radio frequency energy is dissipated directly intera circulating liquid such as tap water. s .y

It is a further object to provide a knovel coaxial line termination which maintain'sastandingwave ratio -in the coaxial line of substantially unity.

Inone aspect, the -termination comprises a coaxial line having an outer conductor andfan inner conductor,v the outer and inner conductors of ythe'termination beng'connected to the outer and inner conductors, respectively, of a coaxial line to be terminated. The termination may be considered as havinganinput end section, an intermediate impedance transformer section and a terminal end section. In the input endy section, the space betweenv the inner and outer conductors is filled with ailow/loss dielectric material 'such as polyethy1ene,' 'polysty'rene,`

or Teony In the intermediate impedance transformer section, the space between the inner and outerconductor is in part occupied by the same low loss dielectric ma-V terial, and in part by a circulating liquid such as tap water. The dielectric material is internally 'tapered so that at the input end of the Aintermediate section, all of the space between the inner and outer conductors is` termination depends on the dielectric constant K of the water circulating therein. The characteristic impedance may therefore vary in accordance with the chemical and physical properties of the water being used. Nevertheless, since the same water is used in the intermediate section as is used in the terminal end section, a good impedance match is always maintained between the input end section and the terminal end section. The radio frequency energy is dissipated directly in the circulating, tap water, and therefore heat exchanger equipment in the usual sense is not employed. s

These and other objects and aspects of the invention will be apparent to those skilled in the art from the following more detailed description taken in conjunction with the appended drawings wherein:

Figure 1 is a sectional view taken longitudinally thru a coaxial line termination constructed according to teachings of this invention; and

Figure 2 is a transverse section taken on the line 2--2 of Figure l.

The drawings show a coaxial line termination oonnected to a conventional coaxial line 10 adapted to carry high radio frequency energy which may originate in ya radio transmitter. Line 10 has an outer conductor 11 and an inner conductor 12, the space between the conductors 11 and 12 being occupied by air. The coaxial line 10 may be a standard 51.5 ohmline. conductor 11 of a coaxial line 10 is securedby any suitable means to the outer conductor 13 of the termination. The inner conductor 12 of the coaxial line 10 is connected to the inner conductor 14 of the termination. The inner conductor 14 is hollow except for the end which is connected'to the inner conductor 12 of the coaxial line 10. The inner conductor 14 is perforated with apertures 15 topermit the circulation of Water in a manner which will become apparent as the description proceeds.

4 outer and inner conductors, which. are fixed, and. upon the,J

For the purpose of facilitating the description, the termination will be considered as including an input end section 17, an intermediate transformer section 18 and a terminal end section 19. The space between the outer conductor 13 and the inner conductor 14 in the input end section 17. is completely tilled with a low loss dielectric material 20 such as "polyethylene, polystyrene, or Teilonf VThese materials have a dielectric constant K in the order of 2. It will be noted that the inner conductor 14 has a smaller diameter than the inner contuctor 12 of the coaxial line 10 to which it is connected. The diameter of the inner conductor 14 is smaller than the diameter of the inner conductor 12 yby an amount which exactly compensates for the transition of the air dielectric (K=l) in the coaxial line 10 to the solid dielectric (K1-2) in the input end section 17. Stated another way, the characteristic impedance of the input end section 17 is the same as the characteristic impedance of the coaxial line 10 (51.5 ohms, for example) so that radio frequency energy is passed into the termination substantially without reflections.

The solid dielectric member 20 extends into the intermediate transformer section 18, and that portion of member 20 within the section 18 is provided with an internal taper so that the member 20 has a feather edge at 22. The length of the intermediate impedance transformer Asection 18 is long compared with the wavelength of the radio frequency energy being dissipated, so that the impedance transformation is accomplished substantially without reflections.

The terminal end section 19 constitutes a coaxial line,

with a lossy dielectric material in the form of tap water circulated between the outer conductor v13 and the inner conductor 14. The characteristic impedance of the terminal end section 19 depends upon the dimensions of the I s Patented Anr-` Y7, 1.359

The outer` assnesa dielectric constant of the tap water circulated therethru. The dielectric constant of water is a ligure in the order of 90, but the value varies considerably with the temperature of the water. The dielectric constant of tap water also varies from one locality to another depending upon the chemicals which are present in the water. Therefore, the characteristic impedance of the end section 19 may vary considerably in the region of about 8 ohms.

The terminal end section 19 is electrically short-circuited by'means of a conductive annulus 23 connecting the outer conductor 13 with the inner conductor 14. Tap water is directed into the inner conductor 14 at 24 from which it passes thru the hollow inner conductor 14 and out thru the apertures or perforations 15 to the space between the inner conductor 14 and a solid dielectric 20. The water ow, as shown by the arrows in the drawings, continues to the terminal end section 19 and out the discharge pipe or port 25.,

The direction of water o'w may be reversed from that shown and described. The direction of water flow should preferably be such that air bubbles which appear in the water cannot be trapped at any point between the outer conductor 13 and the inner conductor 14. With the direction of water ow shown in the drawing, the termil nation should be mounted in such a way that the discharge port 25 is elevated relative to the rest of the termination so that air bubbles will tend to go out the discharge port 25. On the other hand, if the terminal end section 19 is lower than the balance of the termination, the direction of water flow should be reversed so that bubbles will be carried by the stream into the interior of the inner conductor 14. There is no electrical energy inside the inner conductor 14, and therefore the electrical characteristics of the termination cannot be affected by the presence of air bubbles inside the inner conductor.

In the operation of the coaxial line termination, radio frequency energy is applied from the coaxial line to the input end section 17 without attenuation or reflections by reason of the fact that the diameter of the inner conductor 14 is reduced to compensate for the change in the dielectric constant of the low loss dielectric member as compared with air. The intermediate section 18 serves as an impedance transformer to match the impedance of the terminal end section 19 to the impedance of the input end section 17. The impedance transformation results from the gradual transition from the solid dielectric 20 to the liquid dielectric. The transition from the characteristic impedance of the input end section 17 as determined by the solid dielectric 20, to the characteristic impedance of the terminal end section 19 as determined by the water dielectric is gradual so that substantially no radio frequency energy is reflected back into the coaxial line 10. The construction is such that no matter what the characteristic impedance of the terminal end section `19, the intermediate impedance transformer section 18 will provide a proper impedance transformation since the samewater flows in both the intermediate section 18 and the end section 19.

The radio frequency energy llowing into the intermediate section 18 and the terminal end section 19 is dissipated directly in the circulating water because of the relatively high dielectric constant of the water which may be in the order of 90. The radio frequency energy translated into heat in the water causes a maximum heating of the water at the point 22 between sections 18 and 19. However, radio frequency energy is translated into heat at all points in the water between outer conductor 13 and inner conductor 14. As the energy passes thru the end section 19 to the end shorted by the conductive annulus 23, it is continuously attenuated and translated into heat in the water. The electrical energy reaching the shorted end 23 of the termination is reflected back through the terminal end section 19 and the intermediate section 18. As the energy goes back it is further attenuated so that substantially no electrical energy gets back into the. coaxial line 10. 1

The power dissipating capacity of the termination may be increased to any desired value by simply increasing the velocity of the tap water passed thru the termination. 1n this Way, very great amounts of radio frequency energy may be dissipated in the circulating water. The transformer section 18 and the terminal end section 19 of the termination may be made as long as is required to keep the standing wave ratio in the coaxial line 10 within the tolerances desired.

The input end section 17 may be reduced in length as much as manufacturing tolerances will allow. The input end section 17 is included so that an error in making the taper in the dielectric member 20 cannot result in an air space between the inner conductor 14 and the dielectric member 20 at the point' wherethe inner conductor 14 connects with the inner conductor 12 of the coaxial line 10.

The coaxial line termination may be short-circuited at the terminal end as shown by the conductive annulus 23 in the drawing, or the terminal end may be open circuited. The former arrangement, however, is easier to construct and is generally more satisfactory.

Solely by way of example, a coaxial line termination was constructed for connection to a standard 51.5 ohm coaxial line. yThe termination included an outer conductor 13 having an inner diameter of about 3 inches, and an inner conductor 14 having an outer diameter of about inch. The length of the tapered intermediate impedance transformer section 18 was about 29 inches, and the overall length of the termination was about 4 feet. Ten kilowatts of radio frequency energy was supplied to the termination at frequencies in the range between 400 and 900 megacycles. The standing wave ratio in the coaxial line 10 was in the order of 1.08. Ordinary tap water was circulated thru the termination. The termination operated in an entirely satisfactory manner in dissipating the radio frequency energy.

The termination of this invention is useful over a broad band of frequencies by reason of the absence of circuits, such as quarter-wave stubs, which would limit the range of frequencies that can satisfactorily be handled.

What is claimed is:

1. A coaxialvline termination wherein large amounts of radio frequency power may be dissipated comprising, an outer conductor, a hollow inner conductor, a solid internally tapered low loss dielectric member occupying a portion of the space between said outer and inner conductors, and means to apply a liquid thru a path including the interior of said hollow inner conductor and the space between said outer and inner conductors not occupied by said solid dielectric member.

2. A coaxial line termination or load wherein large amounts of radio frequency power may be dissipated comprising, an outer conductor and a hollow coaxial inner conductor, said outer and inner conductors being divided along their lengths into an impedance transformer section and a terminal end section, a solid internally tapered dielectric member in said impedance transformer section, said hollow inner conductor being provided with perforations within said impedance transformer section, said outer conductor being provided with a port in said terminal end section, and means to circulate water in a path including said hollow inner conductor, the perforations in said inner conductor, the space between said outer and inner conductors not occupied by said solid dielectric member and said port in said outer conductor.

3. A coaxial line and a termination therefor wherein large amounts of radio frequency power may be dissipated comprising, a coaxial line having outer and inner conductors with an air dielectric therebetween, a coaxial line termination including an outer conductor connected to the outer conductor of said line and having a hollow inner conductor directly connected at one end to the inner conductor of said line, the inner conductor of said termination having a uniform but smaller cross-sectionaldimension than the inner conductor of said line, a solid dielectric member occupying a portion of the space between the outer and inner conductors of said termination, said member occupying all of the space between said conductors at the point where the termination is connected to the line, and being internally tapered from said point along the termination to a feather edge at a second point on said outer conductor of said termination where none of the cross-sectional area of the termination is occupied by said solid dielectric, the reduction in crosssectional dimension of said uniform inner conductor of said termination being such as to compensate for the change in the dielectric constant of said solid dielectric member as compared with air, and means to apply a liquid to the space between the outer and inner conductors of said termination not occupied by said solid dielectric member, said dielectric member and liquid forming part of an impedance transformer to provide a proper impedance match between said coaxial line and said termination.

4. A coaxial line termination wherein large amounts of radio frequency power may be dissipated, comprising inner and outer coaxial conductors having uniform crosssectional dimensions along their lengths, said termination having an input end and a terminal end, said inner conductor being hollow and provided with perforations near said input end, and said outer conductor being provided with a port near said terminal end, solid dielectric means defining a tapered chamber that is positioned between said inner and outer conductors and that has zero cross-sectional area near said input end and that has full cross-sectional area toward said terminal end, and means to lill said chamber with a liquid, whereby said liquid may be circulated in a path including said hollow inner conductor, said perforations, said chamber, and said port.

5. A coaxial line termination wherein large amounts of radio frequency power may be dissipated, comprising a hollow outer conductor having a uniform cross-sectional dimension along its length, a hollow inner conductor having a uniform cross-sectional dimension along its length coaxially positioned within said outer conductor, said inner conductor being provided with perforations, an internally tapered low loss dielectric member occupying a portion of the space between said outer and inner conductors, and means to insert lossy iluid in the space between said outer and inner conductors not occupied by said dielectric member, said means including a path through said hollow inner conductor, said perforations, and said space.

References Cited in the tile of this patent UNITED STATES PATENTS Measurement, vol. l1, M.I.T. Rad. Lab. Series, published 1948, McGraw-Hill, page 198.

Publication II, Kirchner and Shaw, Coaxial-Type Water Load and Related Power Measuring Apparatus, Proc. of the I.R.E., vol. 3S, January 1947, pp. 84-87. 

